Neptunium n. : A new metallic element, of doubtful genuineness and uncertain indentification, said to exist in certain minerals, as columbite.

Example:
Is Neptunium found in nature or only in a step in plutonium production?

Taxonomy
neptunium, Np, half-life, element, isotopes of neptunium, decay, neutrons, stable isotope, Neptunium 237, isotopes, neptunium series, free encyclopedia, density, americium-241, actinides, reactions, nuclear waste, cm³.

Defination from the web:
... Neptunium (named for the planet Neptune, the next planet out from Uranus, after which uranium was named) was first discovered by Edwin McMillan ...